Not applicable.
Not applicable.
This invention relates to harmonic mitigating devices for electrical power distribution systems and more particularly to a passive harmonic mitigating device for connection between a power distribution system and one or more harmonic-generating loads that reduces the level of harmonic currents flowing into the power distribution system.
Electrical distribution systems used to distribute electrical power to buildings, manufacturing facilities, etc., are often subjected to harmonic currents generated by non-linear loads such as electronic equipment, adjustable speed drives (ASD), uninterruptible power supplies (UPS), power rectifiers, etc. Among other harmonics, it is known that these loads are capable of routinely causing 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc. harmonics in the power distribution system.
As well known in the art, load generated harmonic currents cause many problems in power distribution systems including increasing the voltage total harmonic distortion level, reducing the electromagnetic compatibility of the loads, reducing reliability of the power distribution equipment, increasing power losses, reducing system power factor, etc.
Prior art systems for mitigating harmonic currents have included configurations that can be grouped into many different categories. One important category of mitigating system is generally referred to as a passive filter network. Passive networks are systems wherein devices within the networks are selected to configure filters based on desired operating characteristics and then, as the name implies, the networks themselves operate, independent of controllers or the like, to reduce harmonics.
One type of passive filter network includes a plurality of trap filters that are individually tuned to eliminate specific harmonics. For instance, because the 5th, 7th and 11th harmonics typically have the largest magnitudes, one exemplary passive network includes three trap filters arranged in parallel between the source and load, one filter for each of the 5th, 7th and 11th harmonics. Often the filter that mitigates the 11th harmonic will be designed to mitigate higher order harmonics as well. Each filter includes a reactor including inductive windings disposed on a core, capacitors and typically resistors wherein the capacitors and resistors are arranged in either a delta or wye configuration. Another exemplary passive filter network includes three trap filters arranged in series between the source and load, each filter tuned to mitigate specific harmonics and including a separate core, inductive windings, resistors and capacitors.
These multi-filter networks are advantageous in that the fluxes generated by the windings are relatively simple and easy to comprehend and therefore the networks are easy to design and construct. To this end, because multi-filter networks include separate cores for each of the trap filters, there is no need to account for mutual inductance between filter windings during design.
Unfortunately, while simple to design and construct, the multi-filter networks require a large number of components including resistors, capacitors, windings and a separate core for each of the filters in the network. Not only are the large number of components expensive but the number of components increases overall space required to house the networks.
In an effort to reduce network size and component related costs, another type of passive filter network has been developed which is referred to generally as a broad band filter network. Instead of requiring separate resistors and capacitors for each harmonic to be mitigated, broadband networks typically include first and second line reactors, a trap reactor and a delta or wye connected capacitive and resistive assembly. The first line reactor includes a separate winding for each of the three supply lines in a three phase system, each winding disposed on a first reactor core and linked to a separate one of the supply lines at a first end and to a separate one of three central nodes at a second end. Similarly, the second line reactor includes a separate winding for each of the three supply lines in a three phase system where each winding is disposed on a second reactor core and is linked to a separate one of the central nodes at a first end and to the load at a second end. Thus, in series between each supply line and the load are separate windings corresponding to each of the first and second reactors. The trap reactor includes a third core on which are disposed three separate trap windings, a separate one of the trap windings linked to a separate one of the central nodes at one end and linked to the capacitive/resistive assembly at the other end.
In this case the first and second line reactors provide large reactance to harmonics traveling along the supply line while the trap reactor is tuned to provide minimal reactance to the harmonics such that the harmonics travel into the trap circuit where they are effectively xe2x80x9ctrappedxe2x80x9d (hence the label xe2x80x9ctrap circuitxe2x80x9d) within the capacitive/resistive network.
While advantageous over the multi-filter designs because component count is reduced appreciably and therefore cost and required volume are reduced, three core broadband filters as described above are disadvantageous in that they still require three separate cores (i.e., a separate core for each of the first, second and trap reactors). Again, any design requiring additional components typically increases overall network cost and space required to house the network.
Recently some single core broadband filter networks have been designed that reduce overall network size appreciably. To this end, U.S. Pat. No. 6,127,743 (hereinafter xe2x80x9cthe ""743 patentxe2x80x9d) teaches a filter network that includes all network windings on a single core. Specifically, the ""743 patent teaches a first set of reactor windings including a separate first winding for each of the supply lines, a second set of reactor windings including a separate second winding for each of the supply lines wherein a separate one of the second windings is linked in series with a separate one of the first windings between the supply and the load and a set of trap reactor windings that are linked to central nodes between the first and second windings of each line. As in the case of three core broad band networks described above, the ""743 patent network also includes a capacitive/resistive assembly linked to the trap reactor windings. Importantly, the ""743 patent teaches that the first and second windings are disposed on the core in opposite orientations (i.e., the first winding in each series is in a first orientation and the second winding in each series is in an opposite orientation). The ""743 patent teaches that this opposing orientation is necessary in order to minimize the voltage drop across the filter network while still mitigating supply line harmonics.
Thus, the ""743 patent claims that the networks disclosed therein have many advantages and it would be advantageous to have other network configurations that could provide similar advantages.
In addition, while the ""743 patent advantageously reduces the core material required to configure a workable network and therefore reduces system costs, unfortunately, the task of designing and constructing finely tuned single core networks is exacerbated by the fact that the inductances between the single core windings become relatively complex due to mutual inductances between the separate first, second and trap windings. In some cases the extra design and construction costs needed to account for the mutual inductances may be greater than the costs associated with the savings in core material. Thus, it would be advantageous to have a filter network configuration which has some of the advantages associated with a reduced number of cores and components while being characterized by inductance parameters that facilitate a simpler design.
It has been determined that, despite teachings in the ""743 patent that line windings have to be oppositely disposed on a core to achieve desirable harmonic mitigating results without excessive voltage drop, the line windings can instead be disposed so as to have the same polarity and still achieve desired harmonic mitigation with an acceptably low voltage drop. To this end, it has been recognized that by aligning line windings with the same polarity in first and second line reactors, the combined reactance of the series windings cooperates to block harmonics such that some of the harmonics are blocked from the load while other harmonics can be forced into a trap filter. Because the reactances (i.e., the winding fluxes) combine instead of cancel, the total inductance within the line can be reduced by reduction in copper material utilization and the voltage drop can be held to an acceptably low value (e.g., 5% of a drive rating). In fact, it has been determined that with proper component selection, operating characteristics that are essentially identical to the characteristics achievable via the ""743 patent network can be obtained via a network configuration including same polarity line windings in the line portion of the filter network.
In addition, it has been recognized that a compromise between networks that include three separate cores which are bulky and expensive but relatively easy to design due to no mutual couplings between windings and networks including a single core which are relatively less bulky and less expensive to configure but are more difficult to design because of mutual coupling that has to be understood and accounted for can be struck where two cores are used, a first core including two of three network windings and a second core including the third of the three network windings. In this case winding polarities of the two common core windings may be the same or opposite if components and configurations are selected properly. Where the common core windings are disposed in opposite polarities the teachings of the ""743 patent are applicable.
An exemplary embodiment of the invention includes an apparatus for mitigating harmonic currents generated by a load connected to a power distribution source via a supply line. In one embodiment the apparatus comprises first and second magnetic cores, first and second series line windings linked between the supply line and the load and at least one trap circuit including a series linked capacitor and trap winding linked at an intermediate node between the first and second line windings and an output connected to a second line. Here, two of the first, second and trap windings are disposed on the first magnetic core and the third of the windings is disposed on the second core.
In some embodiments the first and second line windings are disposed on the first core and may or may not have the same polarity. In other embodiments the trap winding and one of the first and second line windings is disposed on the first core. The trap circuit may include a capacitor in series with the trap winding between the first and second lines.
The source and load may be three phase. In the case of a three phase system, the first line winding includes a separate first line winding for each phase, the second line winding includes a separate second line winding for each phase and the trap winding includes a separate trap winding for each phase.
The invention also includes an apparatus for mitigating harmonic currents generated by a load connected to a power distribution source via a supply line where the apparatus comprises at least one magnetic core including a first core, first and second line windings in series between a supply line and the load such that the first and second line windings are disposed on the first core so as to have the same polarity and at least one trap circuit including a series linked capacitor and trap winding linked to an intermediate node between the first and second line windings with an output connected to a second line with the trap winding disposed on the at least one core. Here a reactance to harmonic currents between the load and the source is increased by the summation of fluxes generated by the line windings and a selected portion of the harmonic currents is diverted through the trap circuit.
The at least one core may include a second core and the trap winding may be disposed on the second core. In the alternative, the at least one core may consist of the first core. Again, in this case the system may be three phase and, in that case, the first line winding includes a separate first line winding for each phase the second line winding includes a separate second line winding for each phase and the trap winding includes a separate trap winding for each phase.
The invention also includes a method for mitigating harmonic currents generated by a load connected to a power distribution source via a supply line, the method comprising the steps of providing first and second magnetic cores, providing first and second series line windings linked between the supply line and the load, providing at least one trap circuit including a series linked capacitor and trap winding linked at an intermediate node between the first and second line windings and an output connected to a second line and disposing two of the first, second and trap windings on the first magnetic core and the third of the windings on the second core.
The step of disposing may include disposing the first and second line windings on the first core with the same or opposite polarity. In the alternative, the step of disposing may include the step of disposing the trap winding and one of the first and second line windings on the first core.
In addition, the invention also includes a method for mitigating harmonic currents generated by a load connected to a power distribution source, the method comprising the steps of providing at least one magnetic core including a first core, providing first and second line windings and a trap winding, disposing the first and second windings on the first core so as to have the same polarity, linking the first and second line windings in series between a supply line and the load, disposing the trap winding on the at least one core, and linking the trap winding to an intermediate node between the first and second line windings with an output linked to a second line.
Here again, a reactance between the load and the source is increased by the summation of fluxes generated by the line windings and a selected portion of the harmonic currents is diverted through the trap circuit. The at least one core may include only the first core.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.